The PTAP Sequence within the p6 Domain of Human Immunodeficiency Virus Type 1 Gag Regulates Its Ubiquitination and MHC Class I Antigen Presentation

Endogenous peptides presented by MHC class I (MHC-I) molecules are mostly derived from de novo synthesized, erroneous proteins, so-called defective ribosomal products (DRiPs), which are rapidly degraded via the ubiquitin-proteasome pathway. We have previously shown that the HIV-1 Gag protein represents a bona fide substrate for the DRiP pathway and that the amount of Gag-DRiPs can be enhanced by the introduction of an N-end rule degradation signal, leading to increased MHC-I presentation and immunogenicity of Gag. Based on these findings, we sought to identify a naturally occurring sequence motif within Gag that regulates its entry into the DRiP pathway. As the PTAP late assembly domain motif in the C-terminal p6 domain of Gag has been shown to negatively regulate the ubiquitination of Gag, we analyzed the correlation between ubiquitination and MHC-I presentation of PTAP-deficient Gag. Intriguingly, mutation of PTAP not only reduces the release of virus-like particles, but also increases ubiquitination of Gag and, consistently, enhances MHC-I presentation of a Gag-derived epitope. Although the half-life of the PTAP mutant was only mildly reduced, the entry into the DRiP pathway was significantly increased, as demonstrated by short-term pulse-chase analyses under proteasome inhibition. Collectively, these results indicate that, besides driving virus release, the PTAP motif regulates the entry of Gag into the DRiP pathway and, thus, into the MHC-I pathway. Although there are no naturally occurring PTAP mutants of HIV-1, mutations of PTAP might enhance the immunogenicity of Gag and, thus, be considered for the improvement of vaccine development.